Giggs, 40, who is the most decorated player in United's history, said he had lived his dream as a player and was thrilled to become Van Gaal's second in command.

The Welshman made 963 appearances for United - the most any player has made for one club - and won 34 trophies, including 13 Premier League titles, more than anyone else, and the Champions League twice, in 1999 and 2008.

He debuted aged 17 in March 1991 and played his final game when, as interim manager, he brought himself off the bench in the 3-1 win over Hull.

In an open letter to supporters on United's website, Giggs said he felt was fortunate to have enjoyed such a career and thanked former manager Sir Alex Ferguson.

He wrote: "Today is a fantastic day for Manchester United. Louis van Gaal is a great appointment and let me begin by telling you how delighted I am to be working with someone of his calibre. His credentials are second to none and I'm positive the club will thrive under his leadership over the coming years.

"I would also like to take this opportunity to announce my retirement from professional football and embark upon a new and exciting chapter in my life, as assistant manager of Manchester United. I am immensely proud, honoured and fortunate to have represented the biggest club in the world 963 times and Wales 64 times.

"My dream was always to play for Manchester United, and although it saddens me to know I won't be pulling on a United jersey again as a player, I have been lucky enough to have fulfilled that dream playing with some of the best players in the world, working under an incredible manager in Sir Alex Ferguson, and most of all, playing for the greatest fans in world football. I have always felt and appreciated your support.

"I want to also give a huge thanks to the backroom staff and support teams we have and have had at Manchester United over the years. I would not have achieved the success I have without your continuing dedication and commitment to creating the best environment to enable the players to thrive. I would not have won 34 trophies in my career without you. I would also like to say a special thanks to my friends and family for all your love and support.

"For me, today is a new chapter filled with many emotions - immense pride, sadness, but most of all, excitement towards the future. United fans I hope will share and echo my belief that the club, the management and owners, are doing everything they can to return this great club to where it belongs, and I hope to be there every step of the way. To the greatest fans in world football, thank you, I have loved every minute of playing for you and representing the biggest and best club in the world."

United won't retire Giggs shirt

Manchester United will not retire their No.11 shirt following Ryan Giggs' decision to hang up his boots.

Giggs, who made a record 963 appearances for the club, hung up his boots to concentrate on his new role as Louis van Gaal's assistant manager, but the club will still reallocate the former midfielder's jersey number next season.

While retiring shirt numbers is a common practice in football, United will not follow the tradition. They passed on their number seven shirt, worn by club greats such as Bryan Robson, Eric Cantona, David Beckham and Cristiano Ronaldo, and will do the same with the No.11 shirt.

Giggs' retirement means the club has four key shirt numbers up for grabs next season; No.11, No.7, No.9 and No.5, following their decision not to offer Rio Ferdinand a new contract this summer.

Giggs' 20-minute cameo appearance in the last home game of the season, a 3-1 win against Hull two weeks ago, has turned out to be his final act on the pitch for the club.

He had been in interim charge for the final four games of the season but, although held in high regard by the United hierarchy, was not considered as a possible successor because of his lack of experience.

United were keen to keep Giggs, however, with the Welshman saying he has plenty to learn from Van Gaal.

Giggs added: "I am thrilled to have the chance to serve as assistant manager. Louis van Gaal is a world-class coach and I know I will learn a lot about coaching from being able to observe and contribute at such close quarters. Manchester United has been a huge part of my life and I'm delighted to be able to continue that relationship in such a key role."

Executive vice-chairman Ed Woodward paid tribute to Giggs, who he described as a United legend whose feats will never be repeated, and branded the Welshman the best British footballer of the last quarter of a century.

Woodward said: "Ryan Giggs has been one of the greatest players ever to wear the red shirt of Manchester United. What he has achieved will never be equalled in the English game. In the way he played, he was the embodiment of a Manchester United player - fast, skilful, entertaining and determined to win by playing exciting football.

"His progress from the ranks of the famous Class of '92 to the most decorated player in this or any club's history is down to his hard work, ability and attitude. He is at the start of an exciting new chapter in his and the club's life but now is the right moment to reflect on the achievements of the finest British player of the last 25 years."

Woodward said he believes Giggs and Van Gaal will prove an excellent team, combining a belief in progressive football with a knowledge of the United traditions.

He stated: "I'm delighted that Louis has chosen Ryan as his assistant. Ryan's association with the club spans over two decades and his knowledge and stature will be of great use to Louis.

"In addition, this is a fantastic opportunity for Ryan to learn his trade alongside a world-class manager whose attacking instincts and belief in youth are tailor-made for Manchester United."